THE EVOLUTION OF THE V - K-COLORS OF SINGLE STELLAR POPULATIONS

Models of evolutionary population synthesis of galaxies rely on the pr operties of the so-called single stellar populations (SSP). In this pa per, we discuss how the integrated near-infrared colours, and especial ly V-K, of SSPs evolve with age and metallicity. Some of the uncertain ties associated with the properties of the underlying stellar models a re thoroughly discussed. Our models include all the relevant stellar e volutionary phases, with particular attention being dedicated to the a symptotic giant branch (AGB), which plays a fundamental role in the ev olution of the near-infrared part of the spectrum. First, we present t he effects that different formulations for the mass-loss rates produce on the final remnant mass (i.e., on the initial-final mass relation), and hence on the AGE-termination luminosity and the relative contribu tion of these stars to the integrated Light. The results for the evolu tion of the V - K colour are very different depending on the choice of the mass-loss prescription; the same is true also for the B - V colou r in the case of low-metallicity SSPs. Secondly, we describe the chang es occurring in the integrated colours at the onset of the AGE and red giant (RGB) branches. According to the classical formalism for the AG E evolution, the onset of this evolutionary phase is marked by a colou r jump to the red, the amplitude of which is shown here to be highly d ependent on the metallicity and mass-loss rates adopted in the models. We then consider the effect of the overluminosity with respect to the standard core mass-luminosity relation that occurs in the most massiv e AGE stars. Different simplified formulations for this effect are tes ted in the models; they cause a smoothing of the colour evolution in t he age range at which the AGE starts to develop, rather than a splitti ng of the colour jump into two separate events. On the other hand, we find that a temporary red phase takes place similar to 1.5 x 10(8) yr after the RGB develops. Thanks to the transient nature of this feature , the onset of the RGB is probably not able to cause marked features i n the spectral evolution of galaxies. We then discuss the possible rea sons for the transition of V - K colours (from similar to 1.5 to 3) th at takes place in LMC clusters of SWB type IV. A revision of the ages attributed to the single clusters reveals that the transition may not be as fast as originally suggested. The comparison of the data with th e models indicates that the transition results mainly from the develop ment of the AGE. A gradual (or delayed) transition of the colours, as predicted by models which include the overluminosity of the most massi ve AGE stars, seems to describe the data better than the sudden colour jump predicted by classical models.